The present invention relates to a method of heating and processing an optical fiber preform which is performed before drawing a vitrified optical fiber preform to form an optical fiber comprising a core portion and a cladding portion covering an outer circumference of the core portion, and to an apparatus used for the same.
More specifically, the present invention relates to a method of heating and processing an end of an optical fiber preform which processes into a predetermined shape an end of an optical fiber preform used for drawing an optical fiber, and to an apparatus for heating and processing an end of an optical fiber preform used for the same.
Optical fibers are being widely used for many applications such as optical communication and optical measurement.
For example, a single mode silica-glass optical fiber has a core having a diameter of 10 xcexcm, a cladding having a diameter of 125 xcexcm formed on the outer circumference of this core, and a resin covering coated on the outer circumference of this cladding. A dopant for raising a refractive index is introduced in the core, whereby the refractive index of the core is made higher than a refractive index of the cladding.
Such an optical fiber is formed by heating and drawing an optical fiber preform. An optical fiber preform has a core portion corresponding to the core of the optical fiber and a cladding portion corresponding to the cladding of the optical fiber.
When heating and drawing a transparent vitrified optical fiber preform comprising a core portion and a cladding portion covering the outer circumference thereof to produce for example an optical fiber having a core and a cladding described above, the heating is started from the cladding portion located at the outer circumference of the core portion. In an initial stage of the heating, however, usually the core portion does not extend up to the tip of the optical fiber preform. If the core portion inside the optical fiber preform is not exposed from the tip of the optical fiber preform and does not exhibit the shape of the melted and deformed portion at the time of drawing, even if the preform is drawn, a normal optical fiber comprising a core and cladding will not be immediately formed. For this reason, when heating and drawing an optical fiber preform, it is necessary to change the shape of the end of the optical fiber preform to the shape of the melted and deformed portion at the time of drawing in advance. If heating and drawing an optical fiber preform with an end shaped to that of the melted and deformed portion at the time drawing in this way, a normal single mode optical fiber having for example a core with a diameter of 10 xcexcm and a cladding with a diameter of 125 xcexcm formed on the outer circumference of this core can be formed without wasted drawing.
In this way, when drawing an optical fiber preform, as a pretreatment, treatment to process the end of the optical fiber preform to a preferred shape for drawing, for example, the shape of the melted and deformed portion at the time of drawing, becomes necessary.
In this specification, this work will be referred to as an end heating and processing method of the optical fiber preform, and the apparatus used for this treatment will be referred to as an end heating and processing apparatus of an optical fiber preform.
How quickly and efficiently the core portion is made to be exposed from the tip of the optical fiber preform is a key point for improvement of the operating rate of a heating furnace and other equipments.
If heating and processing the end of an optical fiber preform at a high speed, however, since a viscosity of the core portion containing the dopant for raising the refractive index and the viscosity of the cladding portion are different, the drawing is not carried out in accordance with the ratio of the outer diameters of the core portion and the cladding portion and therefore the diameter of the optical fiber can be varied. If the variation of the diameter of the optical fiber becomes large, the thickness of the coating when coating an ultraviolet curing resin or the like on the outer circumference of the drawn optical fiber (outer circumference of the cladding) will become uneven and the coating will become defective leading to breakage of the optical fiber.
In order to prevent such the disadvantage, heretofore a method of machining the tip of the optical fiber preform by a glass lathe to expose the core portion from the end of the optical fiber has been attempted. In this method, however, scraps produced when machining the surface of the optical fiber preform will sometimes deposit at the end of the optical fiber preform. The deposited scraps or the like adversely influence the drawing of the optical fiber preform and sometimes cause also defects in the outer diameter of the drawn optical fiber.
On the other hand, the optical fiber preform as a whole is held at or heated to a temperature of 1200 to 1300xc2x0 C. in the heating furnace after the transparent vitrification and then taken out of the heating furnace into the ambient atmosphere of an ordinary temperature for producing the drawn optical fiber or for storage before drawing. If the optical fiber preform is taken out of the heating furnace into the atmosphere and the surface of the optical fiber preform is rapidly cooled, however, minute strain sometimes remains at the surface of the optical fiber preform. This residual strain tends to lower the strength of the optical fiber after drawing.
An object of the present invention is to provide an end heating and processing method of an optical fiber preform capable of eliminating minute strain remaining in the optical fiber preform and shortening a startup time of a drawing process of an optical fiber.
Another object of the present invention is to provide an end heating and processing apparatus of an optical fiber preform used for the end heating and processing method of an optical fiber preform.
The end heating and processing method of the optical fiber preform of the present invention includes a step of processing an optical fiber preform by heating and melting an end of a vitrified optical fiber preform comprising a core portion and a cladding portion formed on an outer circumference thereof to process the end having a shape for drawing as an optical fiber.
Preferably, the optical fiber preform processing step has an optical fiber preform positioning step of positioning the end of the optical fiber preform in the vicinity of a heating portion for heating the end of the optical fiber preform, an end processing step of heating the end of the optical fiber preform to process the related end to the shape of a melted and deformed portion at the time of the drawing, and an unnecessary portion elimination step of eliminating an unnecessary portion of the end processed portion obtained by heating and melting the optical fiber preform processed to the predetermined shape.
More preferably, the method further includes, after the unnecessary portion elimination step, an optical fiber preform end cooling step of blowing an inert gas to the end of the optical fiber preform remaining after the elimination of the unnecessary portion to cool the same.
More preferably, in the end processing step, the end of the optical fiber preform is processed so that a length from a parallel portion to the end of the optical fiber preform becomes a length, to whereby approach a startup time of the drawing process of the optical fiber the shortest time.
More preferably, the method further includes, after the optical fiber preform processing step, a temperature lowering step for lowering the heating temperature of the optical fiber preform to a temperature whereby thermal strain does not occur in the optical fiber preform even in an air atmosphere.
More preferably, in the temperature lowering step, the optical fiber preform as a whole is heated to 1100 to 1300xc2x0 C., then the heating temperature of the optical fiber preform is lowered to 600 to 400xc2x0 C.
More preferably, in the end processing step, an inert gas sealing gas is made to flow from above to below at the periphery of the end heating portion of the optical fiber preform.
More preferably, before the introduction of the optical fiber preform into a heating region before the end processing step, an inert sealing gas is made to flow from above to below to the end heating portion of the optical fiber preform.
More preferably, in the unnecessary portion elimination step in the optical fiber preform processing step, the heating region is shut off from the outside air, and the unnecessary portion is discharged from the heating region to the outside in a state maintaining the heated state.
More preferably, in the unnecessary portion elimination step in the optical fiber preform processing step, the unnecessary portion is eliminated while leaving an amount corresponding to the optical fiber preform drawn from the start of the drawing of the optical fiber to when a target drawing speed is reached.
An end heating and processing apparatus of an optical fiber preform of the present invention is an end heating and processing apparatus of an optical fiber preform comprising a heating furnace for heating and processing an end of a vitrified optical fiber preform comprising a core portion and a cladding portion covering the outer circumference thereof and a controlling means for controlling the heating and processing, wherein the heating furnace comprises a suspending means located above the related heating furnace, introducing the optical fiber preform into the related heating furnace, and positioning it in elevation position, a first heating means for heating a lower end of the optical fiber preform introduced in the heating furnace, and an unnecessary portion discharging means for discharging the unnecessary portion in the processed portion of the lower end of the optical fiber preform to the outside and wherein the controlling means controls the first heating means to process the end of the optical fiber preform to the shape of a melted and deformed portion at the time of drawing.
Preferably, the apparatus further comprises an unnecessary portion cut away means located below the first heating means and cutting away an unnecessary portion in the processed portion of the lower end of the optical fiber preform.
More preferably, the apparatus further comprises an unnecessary portion holder located below the unnecessary portion cut away means for holding the cut away unnecessary portion.
More preferably, the unnecessary portion holder is provided with a cooling means for cooling the interior of the related unnecessary portion holder.
More preferably, the apparatus further comprises a second heating means located above the first heating means for heating the portion above the lower end of the optical fiber preform introduced in the heating furnace.
More preferably, the apparatus further comprises a coolant discharging means located below the first heating means for blowing a coolant to the melted portion of the end of the optical fiber preform to solidify the end of the optical fiber preform.
More preferably, the controlling means controls the suspending means to position the end of the optical fiber preform at the first heating means for heating and melting the end of the optical fiber preform, controls the temperature of the first heating means to heat and melt the end of the optical fiber preform and process the related end to the shape of the melted and deformed portion at the time of drawing, and controls the unnecessary portion discharging means to discharge the unnecessary portion of the end of the optical fiber preform to the outside.
More preferably, the controlling means controls the unnecessary portion cut away means to cut away the unnecessary portion, has the unnecessary portion held in the unnecessary portion holder, and, when the unnecessary portion is cooled to a predetermined temperature or less, drives the unnecessary portion discharging means to discharge the unnecessary portion to the outside.
More preferably, the controlling means repeats the positioning of the optical fiber preform and the heating and processing until the tip of the core portion of the optical fiber preform exhibits the shape of the melted and deformed portion at the time of drawing.
More preferably, the controlling means performs the step of changing the length from the parallel portion to the end of the optical fiber preform to a length, to whereby approach the startup time of the drawing step of the optical fiber the shortest time.
More preferably, the controlling means controls the drive of the second heating means and the first heating means after the optical fiber preform end processing step to lower the heating temperature of the optical fiber preform to a temperature where thermal strain does not occur in the optical fiber preform even in an air atmosphere.
More preferably, the controlling means controls the drive of the second heating means and the first heating means in the temperature lowering treatment stage so that the optical fiber preform as a whole is heated to 1100 to 1300xc2x0 C. and then the heating temperature of the optical fiber preform is lowered to 600 to 400xc2x0 C.
More preferably, the apparatus further comprises a first sealing gas feeding means for making an inert sealing gas flow from above to below in an internal portion of the first heating means at the time of the end processing of the optical fiber preform.
More preferably, the apparatus further comprises a second sealing gas feeding means for making an inert sealing gas flow from below to above in an internal portion of the first heating means before the introduction of the optical fiber preform into the heating region before the end processing of the optical fiber preform.
More preferably, the apparatus further comprises a partitioning means between the first heating means and a means for discharging the cut away unnecessary portion to the outside for partitioning the space between them in an openable/closable manner and a gas purging means for purging the internal portion of the unnecessary portion holding chamber by an inert gas, and the unnecessary portion holding chamber is located between the partitioning means and the outside.
More preferably, when discharging the unnecessary portion to the outside, the controlling means closes the partitioning means to isolate the first heating means from the outside air, opens the unnecessary portion discharging means to discharge the unnecessary portion held in the unnecessary portion holding chamber to the outside, closes the discharging means to isolate the unnecessary portion holding chamber from the outside air, drives the gas purging means to discharge the outside air from the unnecessary portion holding chamber, and opens the partition portion to connect the first heating means and the unnecessary portion holding chamber.
More preferably, the first heating means is a carbon resistance furnace.